Short version: Don’t think that circles turn sharing sensitive stuff into a good idea.
First of all, Google+ must be applauded for attacking the delicate problem of controlling who sees what of the information to put online. The concept of circles isn’t significantly different from Facebook’s friend groups – but Google+ makes this aspect central both when creating new contacts and when posting new content. This is more than an interface tweak – defaults matter (*). As a result, the semantics of Google+ “friendships” in my opinion already differ from those on Facebook, seeing how people add me to their circles who would never request a Facebook friendship (e.g., unknown friends of friends). However, this new-found privacy may lull people into a false sense of security: Excluding someone in your circles from something you share does not mean they won’t ever see it. They just won’t see it from you.
This may be obvious. It wasn’t for me, so I thought I’ll share a little experiment. Consider you have two friends, A – your friend, and B – your business contact. You share an update which is only intended for your friends, so you exclude the circle including B. A finds this so hilarious he shares it with all of his contacts. These contacts happen to include – guess what – your business aquaintance B. Will B see your update? Yes, she will. You may think “but I told Google+ I don’t want her to see this”. No, you didn’t! You just stated that she won’t be in the set of people who will receive that information from you (**). Any attachment you put on that post will be generously forwarded to B as well. To add further awkwardness, B will see that A was included in the circles of the original post, but she wasn’t, as she’s getting a “A shared [your name] post with you” message. Ouch! In Google+’s defense, you have to say that a) re-sharers get a pop-up telling them that the original post was shared in a limited scope and b) you can disable re-sharing. However, the re-sharer may not care, and the re-share disabling is an option available in a drop-down menu which appears after you post (i.e. much less prominent than the circle selection, and we know what happens to non-default functions).
To conclude: Circles may be misunderstood by some users (e.g., me – I’d love some comments from readers telling me how far off I am here) as serving a purpose which, in fact, can only achieved in a brute-force way via “disable re-sharing”. Solutions would include users being aware of this, Google+ deemphasizing the privacy power of circles, and/or making the “disable re-share” button more salient (or change the semantics to “people I haven’t shared this with are excluded from future re-shares”). Of course: Privacy is hard, and particularly in a social network, giving up some of your privacy is part of why it works. All the more, it’s important for existing privacy features to be clear, otherwise they may cause problems you never would’ve had, had you simply not relied on them at all and just applied the basic heuristic for putting stuff on the internet: Don’t post anything you wouldn’t like to see on the cover of tomorrow’s newspaper!
This post is based on a discussion with Oli T. and stars Crisi K. in the role of B.
(*) Which is why Circle Hack, a Facebook app copying Google+’s interface for assigning friends into groups, doesn’t really close the gap between the two.
(**) It does make sense, not stating X is something different than stating that not X, just be clear that removing a circle does the former.
Update: At least one other person finds this behaviour noteworthy as well (in German)